Multifocal IOLs are an increasingly popular solution for simultaneous correction of both distance refractive error and presbyopia. Multifocal intraocular lenses are commonly known as premium IOLs. Conventional autorefractors and wavefront aberrometers are, however, not designed for measuring multifocal optics, and they typically cannot obtain a meaningful or accurate measurement of a subject's eye implanted with a multifocal IOL.
On the other hand, there is a need for intraoperative and postoperative measurement of eyes undergoing or having undergone cataract surgery and implanted with multifocal IOLs. During cataract surgery with implantation of a premium multifocal IOL, the surgeon may want to measure the IOL's performance such that any correction or adjustment can be done before the conclusion of the surgical procedure. For example, residual astigmatism is common in cataract patients and reduces the benefits of a premium IOL. Should residual astigmatism be detected in an intraoperative measurement, the surgeon can minimize it by readjusting the orientation of a toric IOL or by performing, for example, a technique such as one or more limbal relaxing incisions.
An autorefractors determines the refractive power of an eye over a small optical zone (typically only up to 3 mm in diameter) at the center of the pupil. Commonly, an autorefractor measures the refractive power at a small number of predetermined points (typically 4 points) to calculate 3 parameters of a subject's eye: 1) the spherical refractive error; 2) the magnitude of the astigmatism; and 3) the axis of the astigmatism. Obviously, an autorefractor is not capable of measuring a multifocal lens that has two or more spherical focal powers.
An ophthalmic aberrometer determines the refractive power of an eye by fitting a smooth and continuous wavefront over the entire pupil. Optical power interruption or discontinuities are not resolvable should the measurement data points have a spatial separation comparable to or bigger than the feature of the interruption, e.g., annular rings in a multifocal IOL. Optical power interruption is removed, for example, in the process of wavefront reconstruction with a relatively small number of mathematical terms, such as Zernike or Fourier coefficients. Consequently, a conventional wavefront aberrometer is not capable of measuring a multifocal lens with a sharp change in the spatial profile of optical powers.
Ophthalmic aberrometers and surgical microscopes are both commonly used in corneal and cataract surgery and are typically standalone instruments. Patients and surgeons, in light of recent advancements in cataract surgery such as the use of intraocular lenses for the correction of astigmatism (toric IOLs) and multifocal lenses for correction of both distance and near vision (presbyopia), could benefit from the ability of making intraoperative wavefront measurements. It thus calls for a need to attach an ophthalmic aberrometer onto an ophthalmic microscope.
A standalone ophthalmic aberrometer has its own positioning mechanism to align the instrument axis with the subject eye's visual axis, and thus it is typically too heavy and too big to attach onto a surgical microscope. Also, a standalone ophthalmic aberrometer typically has a short working distance in the range of 30-50 mm, while a surgical microscope typically has a working distance of 200 mm. A short working distance makes it easier for an ophthalmic aberrometer to have a larger measurement range of defocused power. On the other hand, a longer working distance is necessary for a surgeon to perform eye surgery. Therefore, there is a need to redesign the ophthalmic aberrometer in order to be attachable onto a surgical microscope and allow measurement of eyes not only with monofocal IOLs but also multifocal IOLs.
In addition, it is highly desirable for a surgeon to conduct precise microsurgery like cataract surgery with the best optics afforded by the microscope being used. It is thus preferable that the ophthalmic aberrometer shall not interfere in any way with either the working space or the image quality of the microscope.
Another concern for intraoperative wavefront measurement is confirmation of the status of the cornea at the time of the wavefront measurement as the corneal shape is a major factor in the total wavefront error of the eye. Factors, such as the intraocular pressure for instance, may affect the radius/radii of curvature of the cornea. Confirmation of the residual refractive error of the eye at the conclusion of surgery is dependent on the power of the lens implanted and the corneal curvature. Should the corneal curvature(s) not be as they were preoperatively or predicted/estimated to be postoperatively, the surgeon could obtain a less than accurate measurement of the refractive status of the eye during surgery when decisions regarding subsequent interaction, such as lens exchange or limbal relaxing incisions would be made, thereby reducing the efficacy of the intraoperative aberrometer and the final clinical result for the patient. Thus, in the intraoperative measurement of wavefront power, a concurrent measurement of corneal keratometry would be helpful to detect any induced corneal power change in the cornea, such as might be caused by failure to restore the eye to normal intraocular pressure and thus to improve the accuracy of the wavefront measurement or take the altered corneal power into consideration.
We refer to the following patents:    U.S. Pat. No. 6,382,795 May 7, 2002 M. Lai Method and apparatus for measuring refractive errors of an eye    U.S. Pat. No. 6,406,146 Jun. 18, 2002 M. Lai Wavefront refractor simultaneously recording two Hartmann-Shack images    U.S. Pat. No. 6,575,572 Jun. 10, 2003 M. Lai, et al Method and apparatus for measuring optical aberration of human eye    US2005/0241653 Nov. 3, 2005 Van Heugten et al Integrated surgical microscope and wavefront sensor